Many machines use transmissions to couple the output of a prime mover or power source, for example, an internal combustion engine, to a driven element or device such as wheels or a work implement. Traditional transmissions typically included one or more fixed, selectably engageable gear ratios that could increase or decrease the speed of the prime mover and, usually in an inverse relationship, the torque that is output to the driven element. Recently, however, some manufacturers have equipped certain machines with continuously variable transmissions (CVTs) instead of the conventional, gear-based transmissions. A CVT provides an infinite or continuous range of torque-to-speed output ratios with respect to any given input from the prime mover. In other words, the output of the CVT can be increased or decreased across a continuous range in almost infinitesimally small increments. In some embodiments, to improve operator recognition and familiarity, the CVT may be regulated according to a virtual gear methodology that simulates a plurality of virtual gear ratios resembling the actual gear ratios in conventional transmissions.
In some machines, CVTs may function to transmit a retarding power back to the prime mover to slow or limit the machine's propulsion. For example, if a machine is traveling on a declining plane or surface, it may be desirable to redirect some of the output from the CVT that normally is transmitted to the driven elements instead back to the prime mover. The redirected power may be referred to as a retarding power, i.e., power obtained from the machine's driven elements and directed back to the power source where the retarding power may act as a counterpoise. In some instances, the retarding power is harnessed from the momentum or potential energy stored or generated by the driven elements. Because the driven elements, e.g., wheels, have less power directed to them and may be losing power during retarding, their rotation may slow due to the effects of inertia and friction with the ground, allowing the machine to maintain speed on downward slopes or, in some instances, decelerate.
One method for retarding a power source is described in U.S. Patent Publication No. 2010/0137102 (the '102 publication), titled “Retarding Control for a Machine” and assigned to the assignee of the present application. The '102 publication describes a methodology that monitors one or more operator inputs to determine a desired machine speed and compares the desired machine speed with the actual speed of the machine to determine if retarding is necessary. The '102 publication describes that the amount of retarding generated may be determined in part by the difference between the desired speed and the actual speed. The '102 publication may thus be considered a speed-based retarding control.